Sedulous

sedulous \ˈse-jə-ləs\ adjective: 1 : involving or accomplished with careful perseverance {sedulous craftsmanship}, 2. diligent in application or pursuit {a sedulous student}

Etymology: Latin sedulus, from sedulo sincerely, diligently, from sed-, se without + dolus guile

Note the etymology. Something that meant “without guile or deceitful cunning” has come to mean “accomplished with careful perseverance” or “diligent in pursuit.” Makes sense.